After baseline MRI scans of the brain and neuroendocrine testing, primates undergo whole brain radiotherapy in 10 daily fractions, 340 rads each. Six of these primates will be anesthetized with pentobarbital during the irradiation, six will not receive any pentobarbital. Neuroendocrine testing and MRI scans will be done at 3, 6, 12, 18 and 24 months after radiation. Quantitative histology will be done on the capillary bed, glial, and neuronal populations after sacrifice. The purpose of this study is to determine if pentobarbital has protective effects on multiple fractionated whole brain irradiation and to further elucidate the microanatomic substrate of cerebral radiation damage.